The invention relates to a shift lever assembly used to shift a vehicle transmission between a plurality of shift positions. The invention is particularly directed to providing a shift lever assembly with an interlock that operates under pre-load conditions.
Brake transmission safety interlock (BTSI) systems are used to ensure that a vehicle braking system is activated before the transmission can be shifted out of a park position. In the prior art, BTSI systems typically include a shift lever device with a solenoid, a shift cable extending from the shift lever to the vehicle transmission, and an electromechanical device in electrical communication with the vehicle braking system and in mechanical communication with the shift cable. When the vehicle is in park and the vehicle braking system is at rest, the shift lever cannot be shifted out of park, and thus the shift cable cannot shift the transmission out of park. When the vehicle is in park and the vehicle braking system is activated, the shift lever can be moved out of park, causing the shift cable to shift the transmission.
One problem with such prior art BTSI systems is pre-load release. A pre-load release condition exists when an operator tries to shift out of park without first applying the brakes. The locking mechanism in the BTSI system is thus placed under load. As long as the operator maintains the shift load, the locking mechanism will fail to release and allow shifting even if the brakes are subsequently applied. In order to release the locking mechanism, the operator must remove the initial shift load, apply the brakes, and then shift out of park.
An example of a BTSI system with pre-load release is shown in U.S. Pat. No. 5,176,231. This system utilizes a solenoid with a plunger that is used to engage a notch having an S-shaped sigmoidal wall in a shift cable assembly. When the plunger is vertically extended into the notch, the cable does not move, and the transmission cannot be shifted. When the braking system is activated and the plunger is removed from the notch, the cable can move and the transmission can be shifted. Interaction of the plunger and the S-shaped sigmoidal wall of the notch allow shifting out of park under pre-load conditions.
BTSI systems such as these are difficult to package and install because the interlock mechanism is interacting directly with the shift cable. Shift cables are difficult to route through vehicles and the addition of an interlock mechanism on the cable further increases installation difficulties. Also, failures in interlock mechanism such as these can be difficult to service depending on where the interlock is mounted to the shift cable. The present invention provides a shift lever assembly for a BTSI system with an interlock that interacts directly with the selector lever and which can shift out of park even under pre-load conditions. The shift lever assembly is also easy to install and maintain.